Vehicles traveling along a path are sometimes subject to external forces that force the vehicle to change its apparent heading in order to compensate for this external force. Examples include a pilot attempting to maintain a desired ground track in the presence of cross winds, a boat maintaining a straight course in the presence of a cross current, and the like. In so doing, when using guidance to obtain a heading that will lead to an intended target, this direction of travel does not always agree with the vehicle's heading. Piloted vehicles are guided by the driver or pilot to maintain a desired course, correcting for the apparent heading/cross track error. The driver or pilot of the vehicle needs to have some way to correct for this inaccuracy.
The pilot/driver of the vehicle usually relies on one or more heading or guidance instruments to assist in compensating for the apparent heading error and/or cross track error. Using the indications for the guidance, the pilot directs the vehicle, through steering inputs, to maintain a desired ground track, often with varying degrees of success. Inaccurate corrections for this heading error and/or cross track error leads to the vehicle's deviation from the specified course (e.g., actual ground track deviating from desired ground track). Depending upon the application, this can have numerous adverse consequences (e.g., maritime navigation, aerial application of agricultural chemicals, etc.).
For example, in aerial application of agricultural chemicals, such as fertilizers, herbicides, and pesticides (sometimes referred to as "crop dusting") it is very important to fly parallel lines in order to prevent overlap and skip of material applied. Aerial application of agricultural chemicals is the term generally used for the dispensing of chemicals (e.g., fertilizers, pesticides, and the like) to an agricultural field (e.g., field crops, orchards, etc.) from dispensing vehicle (e.g., helicopter). During aerial application, the dispensing vehicle makes numerous sequential passes, dispensing chemicals in a swath across the field in each pass. The helicopter's pilot carefully follows a flight path which ensures that each successive swath over the field is correctly spaced, distance wise, from the adjacent swath in order to avoid gaps or overlaps in coverage. For example, should one swath occur too far from an adjacent swath, the area of the field will not receive an sufficient amount of chemicals (e.g., pesticides, fertilizer, herbicides, and the like). Similarly, should one swath occur too close to an adjacent swath, the overlap area receives excessive amounts of chemicals. This can prove very expensive to the farmer. The crops of the field can be damaged or rendered unusable.
Modern agriculture-type applications use GPS systems to provide course and guidance information. The GPS information is used to control indicators (e.g., Course Deviation Indicator, lightbars, etc.) to allow the pilot/driver to determine correct steering adjustments. However, there exists a particular problem with helicopters. The location of the GPS antenna tends to cause errors in the guidance indications generated therefrom. The position/orientation is determined with respect to the GPS antenna. While in most implementations, the GPS antenna is mounted near the center of the vehicle, the location of helicopter rotor blades may force the location of the GPS antenna outside the arc of the rotor blades at the end of the tail boom, or other point away from the rotor blades. This is to prevent obscuration of the GPS and/or differential correction signals by the spinning rotor blades. Although GPS signals can often be tracked through the spinning rotors, the differential correction signals often cannot (especially those differential correction signals broadcast from satellites). Thus, an integrated GPS/DGPS antenna is often mounted on the tail boom or other appendage, clear of the arc of the spinning rotors.
This creates a problem however, in that the position and guidance information is determined with respect to the location of the antenna. The antenna is located a significant distance away from the center of the helicopter. This leads to significant errors when attempting to correct for flight deviations or adverse winds, or other changes in attitude. For example, helicopters "crab" into the wind to maintain a constant desired ground track in the presence of cross winds. As such, the tail boom is angled away from the center line, "off center" from the desired ground track with respect to the rest of the aircraft. This leads to errors in instrument readings, and unless consciously compensated for by the pilot, errors in course corrections.
Optimal antenna location for guidance applications is near the center of rotation where the vehicle operator is located so the guidance corrections are applicable to the operator's location. The operator can then make the necessary course adjustments based on this information without having to mentally compensate for any heading/cross track error due to the difference between the operator's location, or the vehicle's center of rotation, and the antenna location.
If the optimal antenna location is not practical (i.e., rotor blade interference, obscuration, etc. as described above) then adjustments need to be made to account for the vector between the antenna location and the point where the DPGS determined position is needed for guidance (i.e., the operator's location, the center of spray location, etc.). In further discussions herein this point is referred to as the point of operation. For example, adjustments accounting for the vector between the antenna location and the vehicle's center of rotation. This vector is referred to as a lever arm.
Prior art FIG. 1 is a diagram showing a helicopter 100 flying a ground track 101 (course 000). A differential GPS (DGPS) antenna 103 is mounted on the tail boom of helicopter 100. FIG. 1 also depicts a guidance indicator 150 (e.g., a course deviation indicator, or CDI) as seen by the pilot and a compass heading 110 (e.g., showing a heading of 010) as seen by the pilot.
As shown in FIG. 1, there is a cross wind component 120 (e.g., a 045 wind at 15 knots), the helicopter needs to crab slightly into the wind, in order to maintain the desired ground track. As described above, this crab (e.g., yaw, or applied wind correction angle, with respect to the ground track 101) can offset the guidance antenna location relative to the aircraft's course over ground. This is shown in FIG. 1 as antenna offset 102 (1 meter) and offset angle 104. The offset (the combined effect of antenna offset 102 caused by offset angle 104) can adversely affect the guidance system of the aircraft, giving the pilot an erroneous reading. This is shown by guidance indicator 150 showing the 1 meter offset error with respect to ground track 101. Thus, even though ground track 101 may exactly match a desired ground track, the guidance indicator 150 indicates a course error to the pilot, due to the antenna offset.
It should be noted that this problem is not limited to helicopters alone. For example, other vehicles having GPS antenna locations with a significant offset from the point of operation can experience similar offset induced erroneous readings. One such example would be a ship at sea (e.g., having a stern mounted GPS antenna) having to steer into a current in order to travel a straight line. For example, the ship may have to steer a course of 350.degree. in order to travel in a direction of 360.degree. if there is a mild current from the left. This crab into the current can offset guidance provided by onboard instruments that rely on the ship's heading to provide course information.
Prior art FIG. 2 shows helicopter 100 flying a desired ground track 101 while experiencing a direct head wind 121. In those cases where helicopter 100 is flying straight into or with the wind, there is no error due to crab. As depicted in FIG. 2, helicopter 100 is flying directly into a head wind 121. The direction of the wind 121 is parallel to the ground track 101. Hence, the heading flown by the helicopter 100 matches that of the ground track. There is no significant offset of the GPS antenna, and hence, guidance indicator 150 correctly indicates that helicopter 100 is "on course".
Thus, what is required is a solution that gives correct indications regardless of the attitude of a vehicle (e.g., wind induced yaw, current-induced yaw, etc.). What is required is a solution that properly takes into account the offset caused by the mounting location of a GPS antenna on a vehicle. What is required is a solution that compensates for offset errors due to antenna location and vehicle heading changes. The required solution should be efficiently implemented such that it does not require the installation of additional expensive hardware (e.g., separate GPS and DGPS antennas, inertial measurement systems, etc.). The present invention provides a novel solution to these requirements.